Permanent magnets



Nov. 17, 19770 K. J. sTRNAT ETAL 3,540,945

PERMANENT MAGNETS Original Filed June 5, 196'? 3,540,945 PERMANENT MAGNETS Karl J. Strnat and Gary I. Helfer, Fairborn, NJ., John C. Olson, Dayton, Ohio, and Werner Ostertag, Painted Post, N.Y., assignors to the United States of America as represented by the Secretary of the Air Force Original application June 5, 1967, Ser. No. 644,460, now Patent No. 3,424,578, `dated Jan. 28., 1969. Divided and this application June 11, 1968, Ser. No. 748,889

Int. Cl. H01f N08; C22c 19/00, 3.7/00

U.S. Cl. 14S-31.57 3 Claims ABSTRACT OF THE DISCLOSURE This application is a division of our application, Ser. No. 644,460, tiled June 5, 1967 and now Pat. No. 3,424,578.

BACKGROUND OF THE INVENTION Field of the invention The present invention is predicated on the discovery that certain known intermetallic compounds possess high crystal anisotropy and when subjected to certain processing steps will form permanent magnets.

Representative of such compounds is yttrium pentacobaltide, YCo5. It is to be understood that neither the discovery of YCo5 per se or its property of ferromagnetism is alleged but rather a specific process for treating YCo5 and related compounds to form a permanent magnet.

By way of further explanation and background, the terms ferromagnet and permanent magnet are by no means synonymous. The rst term simply indicates the existence of an ordered arrangement of the atomic magnetic spin moments, a basic physical phenomenon also found in iron and in many other substances. A permanent magnet, however, is a specific device for technological application. While a material has to be ferroor ferrimagnetic to qualify for use in permanent magnets, the properties demanded of the latter are not inherent in any -ferromagnetic material. The material must have a certain combination of basic properties which may be optimized by such measures as adding alloying elements or heat treating. Furthermore, rather complicated processing of the material is usually necessary to produce a permanent magnet with the best possible properties.

With the present invention, the basic property of YCo5, for example, which the present inventors believe they were the iirst to recognize and measure, was its extremely large uniaxial crystal anisotropy. Only with the knowledge of this property and not on the basis of previously published values for Curie point, magnetization, etc., of YCo5 was it possible to predict and to develop process steps whereby permanent magnets could be made of YCO5.

The technology set forth herein wtih respect to making permanent magnets from known ferromagnetic YCo5 represents a distinct and marked advance in the art even "United States Patent O Patented Nov. 17, 1970 as Lodex and Alnico magnets were of patentable merit. With regard to the Lodex magnets, pure iron, pure cobalt, and solid solution alloys of these elements have long been known to exist and to be ferromagnetic. However, the idea of using elongated single domain (ESD) particles for magnets, developed mainly in the last ten years, has resulted in a series of patents on Lodex magnets concerned with the basic principles and properties of such particles as well as details of particle and magnet preparation. In connection with Alnico magnets, ferromagnetic alloys of iron with cobalt and nickel have been known -for many decades. Yet in the course of developing the family of alloys known as Alnicos, various patents were issued covering first the use of such alloys processed in a certain manner for permanent magnets and later minor alterations in composition, heat treatment, sintering, etc., which resulted in better permanent magnets. With such precedents, it is believed that the unusually strong permanent magnets made from YCo5 by the process steps set forth herein are clearly of patentable merit.

Description of the prior art The most pertinent prior art is probably represented by U.S. Pat. No. 3,102,002 to Wallace et al. who describe a group of ferromagnetic materials on compounds prepared `from certain of the lanthanide elements and the transition metals of the first long period which have formulae corresponding to AB5 where A is a lanthanide element or yttrium and B is manganese, cobalt or iron. However, Wallace et al. only describe some basic crystallographic and magnetic properties of these compounds, including YCo5. Furthermore, Wallace et al. make no specific statements about the merits of any of their listed compounds for permanent magnet application. Especially, no mention is made of the key property, high crystal anisotropy, nor is there any reference to the necessity of making powders of the compounds.

SUMMARY OF THE INVENTION The present invention consists essentially of a novel method of preparing a permanent magnetic material and the product resulting therefrom which is characterized by a high saturation magnetization, a reasonably high Curie temperature of several hundred degrees C., and high coercive force. The permanent magnets made by the present method are made from particles with magnetocrystalline anisotropy instead of shape anisotropy, and thereby avoid a number of disadvantages noted of the latter. The outstanding advantage of the permanent magnet material made by the present method is its potential energy product which is 29.2 mg. oe. (1 mg. oe.=106 gaussXoersted) for 100 percent packing of prefectly aligned, single-domain particles and 16.4 mg. oe. for percent density packing. This compares with (a) 9.5 mg. oe. for a platinum and cobalt alloy which is the maximum energy product known for a commercial magnet, (b) 12.5 mg. oe. for the best laboratory magnet of Alnico, and (c) 6.5 mg. oe. for the commercial Lodex (ESD) magnets.

The permanent magnets of the present invention find application is communication equipment, control devices, navigational instruments, auxiliary power generators, etc.

Specic examples include instruments which are based.

on the galvanometer principle, small electrical motors and generators, microwave tubes (in magnetrons and as focusing magnets in traveling wave tubes), biasing magnets for relays, microphones and telephones, and loudspeakers. The use of magnets in motors to replace the conventional stator windings is rapidly gaining acceptance and, while until recently only very small rotating electrical machines were built this Way, permanent magnets are now invading the medium-power motor field. New concepts for equipment to be used on board of air-or spacecraft are presently under study which require strong, large-volume, steady magnetic fields such as magnetohydrodynamic energy converters, devices which would direct the ow of hot plasma or of radiation particles around a space vehicle, and magneto-plasmadynamic engines for space vehicles. While present designs are mostly based on the use of electromagnets, permanent magnets of appreciably larger energy density than presently available combined with good high-temperature performance would simplify designs considerably and reduce the equipment weight.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. 1 is a graph showing the intrinsic coercive force of powders produced by ballmilling plotted as a function of grinding time and particle diameter.

FIG. 2 is a graph showing the magnetization curves of a spherical single crystal of YCo5 measured in the easy and hard directions; and

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is predicated upon the discovery that permanent magnet materials or alloys having a potential energy product which surpasses available permanent magnet material by a factor of over 2.5 can be prepared from (1) a rare earth metallic component selected from the group consisting of yttrium, scandium, lanthanum, cerium praescodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium including mixtures thereof a`nd (2) a second metallic component selected from the group consisting of cobalt, manganese and iron including mixtures thereof. This discovery in turn is dependent upon the recognition and measurement for the first time of the extremely large uniaxial crystal anisotropy of the aforementioned materials as typified by YCo5 coupled with certain process steps. The manner in which the magneto-crystalline anisotropy of YCo5 is measured is set forth in an article entitled Magnetocrystalline Anisotropy of Some Rare- Earth-Cobalt Compounds published in the Journal of Applied Physics, March 1967. The ratio of rare earth metal employed to the second metallic component is 11 to 22 atomic percent rare earth metal to 78 to 89 atomic percent of Co, Fe or Mn.

In general, the permanent magnet materials of the present invention are prepared by melting together the desired amounts of the two general components, such as Y and Co, under a protective noble gas atmosphere or under a vacuum. This step may be effected by arc melting on a cold copper hearth, induction melting in pure alumina crucibles, or containerless levitation melting so as to avoid crucible contamination. The resulting alloy, such as YCo5, can be formed in -3O gram buttons and then crushed and thereafter ground in a ball mill or a vibratory mill. By way of example, after 24 hours ballmilling time, all particles will be smaller than 53 microns, with further milling particles having a diameter as small as .3 to 3 microns can be obtained and effectively used in the present process. (See FIG. 1.) Vibration milling was found to be considerably faster, yielding 1 to Sir-size particles in periods of l/z to hours with charges of YC05 varying from 1 to 25 grams. However, it should be noted that these particles are relatively large compared to the 100 to 1000 A. diameters of the particles required for shape anisotropy particles of comparable coercive force.

Thereafter the particles of the alloy are bonded together which may be effected by several methods. Use of an `organic resin or plastic binder, such as an epoxy resin, is simple and will yield magnets that are sturdy,

easy to shape, and corrosion resistant. They will not be usable at temperatures much above room temperature. High temperature capability may be achieved by sintering or hot-pressing the powder without a binder, or with an inorganic binder such as boron nitride or a metal power. In either case, a magnetic field of at least several kilo-oersteds must be applied before or during the consolidation if oriented magnets with optimum magnetic properties are desired. By way of example, the finely divided particles of YCo5 have been formed into magnets by the following three different methods: (l) The powder was mixed with molten paraffin and the mixture permitted to solidify in a magnetic field of -14 koe.; (2) The powder was compacted to a density of -60 volume percent by means of a hydraulic press in a field of -6 koe. and the resulting magnet soaked in a polystyrene solution and subsequently dried; (3) The powder was stirred into a quickasetting epoxy resin which hardened While the magnet was in a homogeneous 20 koe. field.

The basic principles and considerations of the properties of permanent magnets of this kind were discussed previously in the publication by K. Strant and C. Hoifer, YCo5 A Promising New Permanent Magnet Material, AFML-TR-65-446, Air Force Materials Laboratory, Wright-Patterson Air Force Base, May 1966. These authors determined the magnetocrystalline anisotropy constants of YCo5 at room temperature from magnetization vs. field curves measured on a spherical single crystal of -1 mm. diameter in the direction of the c-axis and normal to it. These curves are shown in FIG. 2. YCo5 was found to have a single easy magnetic direction, the c-axis, and no detectable anisotropy in the basal plane in which the crystal is hardest to magnetize. The maximum applied external field of -45 koe. was far insufficient to saturate the crystal in a direction in the basal plane. A straight line extrapolation off the M vs. H curve for the basal plane yields a saturation field HA @132 koe. (also called anisotropy eld). Based on this extrapolation one can calculate the extremely high anisotropy constant.

The following examples are submitted to illustrate further the invention and not to limit the invention.

EXAMPLE I The metals yttrium (Y) and cobalt (Co), both commercial products of 99.9% nominal purity, were mixed in the weight ratio of 1 to 3.31 (atomic ratio 1:5). Peasize lumps or chips from machining on a lathe were used. In the latter case, it was found advantageous to precompact the charge to prevent loss of chips during melting. The charges of 5 to 10 grams were melted in a levitation furnace (USAF Technical Documentary Report No. ML-TDR-64-90, A Levitation Melting Apparatus for the Preparation of Ultrapure Samples of Reactive Materials by John C. Olson, April 1964) and then cast and cooled rapidly by dropping the melt into a cold, thinwalled porcelain crucible of 5 cm.3 capacity. Temperatures of over 1600" C. were reached during melting, a protective atmosphere of purified argon gas of typically -7 p.s.i.a. pressure was employed to preevnt reaction of the yttrium with oxygen or nitrogen of the air. The resulting ingots were wrapped in tantalum foil, fused into evacuated quartz bulbs, and annealed at a temperature of -1000 C. for 100 hours. This treatment typically resulted in homogeneous alloys and metallographic sections appeared completely single-phase and coarsegrained. The ingots were then crushed in a hardenedsteel mortar until the grains passed through a 60 mesh sleve.

Fifty (50) grams of this coarse powder together with cm.3 of hexane were placed in an alumina milling jar (5" ID x 5" long) with 12 cylindrical alumina pieces (3/4 OD x 1" long) and ballmilled for 50 hours.

The resulting slurry was removed from the jar, dried by letting the hexane evaporate at room temperature, and portions of the powder consolidated into magnets in the following different ways:

(a) 8 grams were placed in a 1/2 diameter cylindrical brass die to be compressed between two hardened carbon-steel pistons inserted axially. An axial magnetic eld was applied with a solenoid surrounding the die. The l'ield was repeatedly turned on and off before pressure was applied, and was then maintained during compacting in an attempt to align the powder particles with their mangetic easy axes parallel to one another. Because the moving pistons also served as pole caps, the eld acting on the sample during compression varied from an initial -6000 oersted to -9,000 oe. A pressure of 51,000 p.s.i. was applied. A cylindrical magnet resulted which had a density of 4.5 g./cm.3 (60% of massive YC05) and a powdery surface and the following magnetic properties: Br=3680 g., MHC=1180 oe., BHC=930 oe.,

(BH)mX=1.1 106 Goe.

(b) 10 g. of the powder were intimately mixed with 2.5 cm.3 of a clear lacquer (Plastiklear No. 225, an acrylic ester resin in colloidal solution containing -12% by weight solids, manufactured by the Illinois Bronze Powder Company, Chicago, 111.), using a porcelain mortar and pestal. The mixture was dried completely in a stream of warm air of -50 C. and re-powdered in the mortar. The powder was then compacted as described before, except that 1" diameter dies and pistons were used and the eld varied from 11,000 to 15,000 oe.

The product was a disc magnet, -1s thick and 62% dense (massive YCo5=l00%) which is mechanically much stronger than the magnet made without a binder. The properties measured in the alignment (=pressing) direction normal to the disc face are Br=3500 g., MHC=960 oe., BHC=750 oe., (HB)maX-=O.7 MGoe.

(c) In an attempt to use an inorganic binder which would not interfere with use of the magnet at elevated temperatures, 10 g., of YCo5 powder were mixed with 2.0 g. of 325 mesh boron nitride powder in the porcelain mortar. The mixture was compressed as in (a). The compact had mechanical strength and cohesion superior to those of the binder-free magnet, but not as good as those of the acrylic-plastic bonded one, with magnetic data inferior to both.

EXAMPLE II In contrast to the procedure of Example I, the YC05 alloy was prepared by fusing the alloying constituents Y and Co in an arc melting furnace having a a watercooled copper hearth and a non-consumable tungsten electrode. Melting was done under a protective atmosphere of either pure argon gas or an argon-helium mixture, the charges weighing between 30 and 60 grams. Each charge was melted and resolidied three to four times to assure god mixing, the buttons were turned over between meltings. The ingot usually broke into several pieces under the thermal stress. They were vacuumannealed for five days at 1100 C. The material prepared in this manner was again crushed to a coarse powder in the steel mortar. 100 g. of powder were ballmilled as described in Example I, but more pieces of grinding medium were used alumina cylinders) and 100 cm.3 of hexane were initially added. Samples of powder were taken from the jar aet regular time intervals for coercive force measurements, and hexane was replenished as needed to maintain the same consistency of the thin slurry.

Specimens for coercive force measurements were made by mixing a small amount (200-300 mg.) of the dried powder with ca. 5 times its weight of an epoxy resin (Allaco Twenty/Twenty), then putting the thick liquid in a 0.4 I.D. x 0.6 cylindrical mold of Teflon, and letting the epoxy harden at a temperature of -70 C. while a magnet field of -l5,000 oe. was applied to orient the particles in axial direction. Measurements of the intrinsic coercive force in this direction as a function of milling time and estimated average partcle size are summarized in FIG. 2. A miximum value of MH=1850 oe. for -5n particles is followed by a drop-off on prolonged milling which is attributed to plastic deformation of the particles which destroys the favorable magnetic symmetry. It is expected that this undesirable overmilling effect can be overcome when the powders are vacuum-annealed at temperatures between 300 and 600 C., or if they are prepared by a technique which avoids plastic deformation such as grinding below room temperature, but for powders prepared by ballmilling at room `temperature coercive forces appear to be limited by it.

EXAMPLE III An alloy was made of 28.3 weight percent yttrium-rich mischmetal (Y-MM) and 71.7 Weight percent Co by arc melting as in Example II and vacuum annealing for 160 hours at 1000 C. The resultant material was -95% single-phase and brittle. It was mortar-crushed to -60 mesh size, a small amount of the powder was imbedded in epoxy resin and this binder was allowed to harden in a magnetic eld as described in Example II. Magnetization curves were measured on this aligned powder sample for the alignment direction and normal to it, using a maximum eld of 45 koe. The magnetization curves resemble those of the YCo5 single crystal (see FIG. 1). The following room temperature data were determined from the measurements on this alloy:

Saturation induction, Bs-9,500 g.

Anisotropy 4lield strength, HA142 koe. Anisotropy constant, K1-|-K;,- -5.4 10'7 erg/cm.3 Density, d=8.06 g./cm.3

From these results the conclusion is drawn that (Y-MM)Co5, as a line particle permanent magnet material, behaves basically like YC05. The upper limit for the energy product is (BH)maX= (Bs/l2)2=22.5 l06 g. oe. The advantage of using the mischmetal instead of pure yttrium is a substantial reduction in the raw material cost with only a small sacrifice in magnet performance.

A typical analysis of Y-rich mischmetal (Y-MM) supplied by the Research Chemical Corporation, Phoenix Ariz., was as follows:

Element: Approx. weight percent Y 57 La 4 Ce 8 lPr 0.5 Nd 3 Sm 3 Gd 3 Dy 4 Ho 1 Er 4 Yb 4 Ca 7 In addition, there were also traces of other rare earths and other elements.

It is to be understood that the above mixtures are simply illustrative of the application of the basic principles of the present invention. By way of example other plastic or resinous materials can be employed as binders in addition to the aforementioned Plastiklear No. 225 and Allaco Twenty/ Twenty. In general the requirements for a binder to be used in dense magnets in the manner outlined above (Example Ib) -are as follows:

The application form must be a true solution or a colloidal suspension of particles having an average size below -0.l/i. The application viscosity must be centipoise (cps.). lPreferably on the order of 1 cp. The boiling point of the solvent should be under C., preferably near 50 C.; the coating left on the particles after evaporation of the solvent must bond them into a solid body under pressures of less than 50,000 p.s.i. at temperatures below 70 C.; the resulting plastic must have very low absorption for Water from the atmosphere.

Another satisfactory commercial product of the same general type as Plastiklear No. 225, is GC Koloid-Clear Acrylic, which like Plastiklear is available in the form of an aerosol container made by the GC Electronics Company, Rockford, Ill. This is a colloidal solution of methyl 4methacrylate in butyl acetate, 40% solids.

On the other hand, a resin to be effective as a binder or matrix for coercive force specimens (see Example Il above) should have the following general properties:

A chemically hardening two-liquid system without a ller material havinga 10W application viscosity 5,000 cp. at 25 C.) and a pot life longer than 15 minutes. It must cure into a hard solid (rather than a rubber-like substance) in no more than 2 hours at room temperature, no more than 30 minutes from the atmosphere.

In addition to Allaco Twenty/Twenty, another commercial product which gives satisfactory results as a binder or matrix for coercive force specimens is Allaco Crystal- Clear also made by Allaco Products, Incorporated, 238 Main Street, Cambridge, Mass.

What We claim is:

1. A permanent magnet having as the active magnetic component an alloy in particle form consisting of component A selected from the group of rare earths consisting of Y, Y-rich mischrnetal, Ce, Pr and Sm, and a second component B selected from the group consisting of:

(a) Co alone, and

(b) Co plus at least one metal selected from the group consisting of Mn and Fe, in the ratio of 11 to 22 atomic percent for component A and 78 to -89 atomic percent for component B.

2. A permanent magnet having as the active magnetic component an alloy in particle form consisting of yttrium and cobalt in the ratio of 11 to 22 atomic percent for said yttrium and 78 to '89 atomic percent for said cobalt.

3. A permanent magnet having as the active magnetic component an alloy in particle form consisting of an yttrium-rich mischmetal and cobalt in the ratio of 1l t0 22 atomic percent for an yttrium-rich mischmetal and 78 to 89 atomic percent for said cobalt.

References Cited UNITED STATES PATENTS 2,813,789 ll/1957 Glaser --123 3,102,002 8/1963 Wallace et a1 75--152 X 3,326,637 6/1967 Holtzberg et al. 75-152 X 3,342,591 9/1967 Gambino et al. 75-152 3,421,889 1/ 1969 Ostertag et al. 75-170 L. -DEWAYNE lRUT'LEDGE, Primary Examiner G. K. WHITE, Assistant Examiner U.S. Cl. XJR. 75-152, 170 

